Synthesis and Properties of Naphthopyrans with a Benzothiophene Ring

New photochromic benzochromene derivatives condensed with a benzothiophene ring were synthesized. The condensation positions of the thiophene ring influenced the photochromism. 7a-Methyl-7aH-benzo[f][1]benzothieno[2,3-b]chromene (1) underwent a two-photon two-step photochromism, while 7a-methyl-7aH-benzo[f][1]benzothieno[3,2-b]chromene (2) showed an ordinary chromene-type photochromic reaction. The yellow color of 1 disappeared upon irradiation with 405 nm light, and a thermally stable oxabicyclo[3.1.0]hexene isomer was reversibly produced. On the other hand, compound 2 turned orange upon irradiation with 366 nm light, and the orange color quickly returned to the original pale yellow in the dark. Upon irradiation with both 366 and >420 nm light compound 2 isomerized to an oxabicyclo[3.1.0]hexene form.